Human Rb(+) AB type red blood cells were modified with methoxy-polyethylene glycol (mPEG) in order to decrease agglutinabilities toward clinically important anti-A, anti-B and anti-D reagents. Attachment of mPEG to membrane proteins of the red blood cell was identified by the shift of well-known blood group active glycoprotein bands such as band 3, 4.5 and PAS-1 on the SDS-polyacrylamide gel electrophoresis. When 4mM concentration of mPEG was added to red blood cells, agglutination was minimum by a blood group typing and microwell agglutination tests. Antibody binding tests showed decreased antibody binding to blood group antigens after mPEG attachment. The decrease of both agglutinability and antibody binding was the result of mPEG attachment to blood group active glycoprotein of the cell membrane. The morphology of red blood cells after mPEG attachment was the usual discocytic cell. Oxygen equilibrium curves of the mPEG-attached red blood cells were similar to unmodified red blood cells. This approach to decrease agglutinability of the red blood cells toward blood group antibodies may be used to develop a universally transfusible blood substitute.